The miniaturization and increased levels of integration of semiconductor integrated circuits have led to a demand for increases in the resolutions of semiconductor exposure devices (hereinafter referred to as “exposure device”). Accordingly, advances are being made in the reduction in the wavelengths of light emitted from exposure light sources. Gas laser apparatuses are being used as exposure light sources instead of conventional mercury lamps. At present, a KrF excimer laser apparatus that emits ultraviolet light at a wavelength of 248 nm and an ArF excimer laser apparatus that emits ultraviolet light at a wavelength of 193 nm are being used as gas laser apparatuses for exposure.
As a current exposure technology, immersion exposure has been put to practical use, for reducing the apparent wavelength of an exposure light source by filling the space between the projection lens of an exposure device and a wafer with a liquid, and changing the refractive index of the space. In the case where immersion exposure is carried out using an ArF excimer laser apparatus as the exposure light source, the wafer is irradiated with ultraviolet light at a wavelength of 134 nm in the liquid. This technology is referred to as ArF immersion exposure or ArF immersion lithography.
The spontaneous oscillation spectral linewidth of a KrF or ArF excimer laser apparatus is as wide as 350 to 400 pm, and therefore a laser beam (ultraviolet light) which is reduced and projected on the wafer by the projection lens in the exposure device exhibits chromatic aberration. As a result, the resolution is dropped. It is therefore necessary to narrow the spectral bandwidth of the laser beam emitted from the gas laser apparatus until the chromatic aberration reaches a level that can be ignored. The spectral bandwidth may be referred to as a spectral width. Accordingly, the spectral width has been narrowed by providing a line narrowing module (LNM) having line narrowing elements in the laser resonator of the gas laser apparatus. Here, the line narrowing elements may be an etalon, a grating, and so forth. A laser apparatus having a narrowed spectral width in this manner is referred to as a line narrowing laser apparatus.